CROSSOVER OF SPECTRAL SCALING IN THERMAL TURBULENCE

The scaling ranges of temperature and velocity fluctuations in thermal ly driven turbulence are studied by analyzing the various contribution s to the equations of motion. The crossover wave number k(B) between B olgiano-Obukhov and Kolmogorov-Obukhov scaling is estimated in terms o f the forcings. By evaluating the thermal and buoyant stirrings and di ssipations of Rayleigh-Benard convection experiments we find k(B) much larger than L-1, the energy-containing scale, but smaller than (10eta )-1, the viscous scale. For computer simulation of randomly thermal dr iven turbulence we find k(B) of the order of L-1. This might explain w hy the Bolgiano-Obukhov scaling was observed in laboratory experiments whereas Kolmogorov-Obukhov scaling was found in computer simulation o f thermally driven turbulence.